Investigators say they weren't able to determine what impelled Adam Lanza to kill his mother in her bed, then drive the short distance to Sandy Hook Elementary School and slaughter 26 children and educators last Dec. 14, and finally kill himself.

A summary investigative report released by the state's attorney's office Monday offered chilling details of 20-year-old Lanza's killing rampage, including 10 minutes of terror inside the elementary school he once attended.

But investigators said they couldn't determine a motive or explanation for his actions. They found, however, mysterious clues.

In fifth grade, Adam Lanza wrote a book that included tales of children being slaughtered and a son shooting his mother in the head.

In the years that followed, he was obsessed with mass murders, assembling articles, photos, books, footage and violent video games, including one in which players gun down students in school. He even kept a spreadsheet ranking mass murders.

The report said Lanza acted alone and did not conspire with others to unleash his bloody attack.

State investigators said their review of computer and social media data showed Lanza had a preoccupation with mass shootings, particularly the Columbine High School shootings in Littleton, Colo., in 1999.

He shot his mother in the head as she lay in bed, the morning after returning from an out-of-town trip without her son, the report said. Lanza then left the house and drove his 2010 Honda Civic to the school, where he shot his way through the locked entrance, killed two administrators and wounded two others before entering two classrooms filled with children, firing his semiautomatic rifle with deadly effect.

Killed first was the principal, Dawn Hochsprung, and school psychologist Mary Sherlach in the north hallway, the report said.

After wounding two others, he walked into the classrooms and killed 20 children and four teachers.

The final shot at the school was fired 63 seconds after the first Newtown police officer arrived at the school. About five minutes later, officers entered the building.

The report gives this account of Lanza's spree:

•The first 911 call to police was received at 9:35:39.

•At 9:36:06, a dispatcher broadcast that there had been a shooting at Sandy Hook school.

•The first Newtown police officer arrived at the school at 9:39:00. Two more officers arrived 13 seconds later. Gunshots were heard.

•At 9:39:34, a police officer encountered an "unknown male" running along the east side of the school.

•At 9:40:03, the last gunshot was heard.

•At 9:44:47, Newtown police officers entered the school.

Lanza, 20, used a Bushmaster XM15-E2S rifle to kill the victims at Sandy Hook school before killing himself there with a Glock 10mm handgun. He killed his mother, Nancy, in her bed with a .22-caliber rifle.

The Connecticut State Police's full evidence file about the shootings, which may total thousands of pages, was not released Monday.

Police found numerous violent video games in the basement of the Lanza home, but most of the people who knew Lanza and were interviewed by police said they had no explanation for his actions, the report says.

The autopsy performed on Nancy Lanza determined that the cause of death was multiple gunshots to the head, the report says. An autopsy of Adam Lanza on the same day revealed that Lanza was 6 feet tall and weighed 112 pounds. No drugs were found in his system.

One month before the shooting, Nancy Lanza sought to buy her son another computer or parts for a computer to build himself, the report said. She was concerned about him and said he hadn't gone anywhere in three months.

She said he would communicate with her only by e-mail, although they lived in the same house. She never expressed fear of Adam or concern about her safety, the report said.

She planned to sell her Newtown home and move to Washington state or North Carolina so her son could go to a special school or get a computer job, the report said. She planned to buy a recreational vehicle where Adam could sleep while the house was being shown to prospective buyers because Adam would not sleep in a hotel, the report says. He refused to leave the home and go to a hotel when the home lost power during Hurricane Sandy in October 2012.

In November 2012, Nancy Lanza wanted to buy her son a CZ 83 pistol for Christmas and prepared a check for that purpose.

At around 5:30 a.m. on Dec. 10, four days before the shootings, Nancy told a friend that Adam had "bumped his head badly," the report says. There was some bleeding, but "he was okay."

Roses with photos of shooting victims are posted on a light post at a makeshift memorial near the main intersection of the Sandy Hook village of Newtown, Conn., on Dec. 19, 2012.(Photo: Julio Cortez, AP)

Nancy left for a trip to New Hampshire and cooked Adam's favorite food before she left. She went to New Hampshire without him and returned Dec. 13 at around 10 p.m., the night before she was killed.

While Nancy Lanza was away, a GPS device in the Lanza home showed that a trip was made from 2:09 p.m. to 2:32 p.m. on Dec. 13, the day before the shootings. The trip was to and from the Sandy Hook area from the Lanza house and was presumably made by the shooter in his black Honda Civic, the report says.

Lanza, it says, appears to have had "no continuing involvement" with the school after he went there as a youth. He attended from 1998 to 2003, first through fifth grades, the report said. He was not assigned to the two classrooms where the shootings occurred.

In 2005, the report says, Lanza was diagnosed with Asperger's syndrome — an autism-spectrum disorder that is not associated with violence — and he lacked empathy for others.

People who knew Lanza describe him "in contradictory ways," the report says. "He was undoubtedly afflicted with mental health problems," the report says. "Yet, despite a fascination with mass shootings and firearms, he displayed no aggressive or threatening tendencies."

The report says that "in some contexts," Adam was viewed as having "above-average intelligence" and, in other contexts, below-average intelligence.

Some people said he had been bullied, but others, including many teachers, said he wasn't.

Nancy and Peter Lanza, who were divorced, said Adam was bullied growing up, the report says. Peter said the bullying was not excessive and concerned Adam's social awkwardness and physical gait.

USA TODAY interviewed four classmates after the shootings, and all said students did not bully him.

The report says Adam "appears to have had few friends growing up."

Adam's father, Peter, saw him regularly until he was 18, the report said. They went hiking and played video games, and went shooting twice. Adam had a cellphone but never used it. His father had to contact him by e-mail.

Adam's relationship with his father deteriorated in the fourth quarter of 2010, the last time the two saw one another. Prior to December 2012, Adam stopped responding to his father's mail and e-mails about getting together, the report says.

One witness said he did not have an emotional connection with his mother. When Nancy Lanza asked him if he would feel bad if anything happened to her, her son replied, '"No,'" the witness said, according to the report.

Other witnesses, though, said Lanza was close to his mother and she was the only person he would talk to.

A person who knew him in 2011 and 2012 said Lanza described his relationship with her as strained because he thought her behavior was not rational.

The report says Lanza often changed clothes during the day, and his mother did his laundry daily. She was not allowed in his room, even to clean, the report says. Lanza disliked birthdays, Christmas and holidays, the report says. He didn't allow his mother to put up a Christmas tree. She explained that by saying Adam had no emotions or feelings, the report says.

Shooting was a family pastime, and Lanza enjoyed shooting and going to a target range with his mother and brother, the report says. He and his mother took a National Rifle Association safety course.

He played violent and non-violent video games often, both solo and online, the report says. His favorite at one point was Super Mario Brothers.

An acquaintance who played a music and dance video game called Dance Dance Revolution with him in 2011 and 2012 said Lanza never mentioned having been bullied growing up. The acquaintance said Lanza enjoyed nature and hiking and sometimes laughed, smiled and made dry jokes.

With the release of the summary report, "the investigation is closed, and no additional release of information or documents by this office is anticipated," the report's author, State's Attorney Stephen Sedensky III, said in a written statement Monday.

He didn't release recordings of 911 emergency calls made on Dec. 14. Their release was recommended by a state Freedom of Information Commission in September, but Sedensky, in an attempt to protect victims' families, has gone to court to stop the release.

The shootings shattered the small, close-knit western Connecticut community and raised several national issues. President Obama went to Newtown on Dec. 16 to comfort families who lost loved ones, then pushed for new gun-control laws. Proposed legislation was not passed by Congress, but Connecticut and some other states implemented stricter gun laws.

The shootings also prompted many communities to take steps to make schools more secure and began debates about the effectiveness of the nation's mental health system.

Release of the summary report followed a months-long debate among Connecticut officials, news outlets, open-government advocates and others over the public's right to know and the pain such a report could inflict on families who lost relatives in the shootings.